Chlorine-containing silazane polymers, process for their preparation, ceramic material containing silicon nitride which can be prepared from them, and their preparation

ABSTRACT

The invention relates to novel chlorine-containing silazane polymers, to their preparation, their further processing to ceramic material containing silicon nitride, and this material itself. In order to prepare the chlorine-containing silazane polymers, oligosilazanes of the formula (I) ##STR1## in which a &gt;0, b≧0 and n is about 2 to about 12, are reacted with at least one of the chlorosilanes Cl 2  R 4  Si--CH 2  --CH 2  --SiR 4  Cl 2 , Cl 3  Si--CH 2  --CH 2  --SiR 5  Cl 2 , R 6  SiCl 3  or R 7  SiHCl 2  at 30° C. to 300° C., where, independently of one another, 
     R 1  and R 2  =H, C 1  -C 6  -alkyl or C 2  -C 6  -alkenyl, 
     R 3 , R 4 , R 5 , R 6  and R 7  =C 1  -C 6  -alkyl or C 2  -C 6  -alkenyl and 
     where if b=0 the reaction with R 7  SiHCl 2 , if R 7  =C 1  -C 6  -alkyl and none of the other chlorosilanes is present, is to be excluded. 
     The chlorine-containing silazane polymers according to the invention can be converted by reaction with ammonia into polysilazanes, which in their turn can be pyrolysed to form ceramic materials containing silicon nitride.

This is a division of our copending application Ser. No. 07/444,025,filed Nov. 30, 1989, now U.S. Pat. No. 5,032,663.

The invention relates to novel chlorine-containing silazane polymers, totheir preparation, their further processing to ceramic materialcontaining silicon nitride, and this material itself.

The pyrolysis of polysilazanes to ceramic material containing siliconnitride has already been described in the literature (R. R. Wills etal., Ceramic Bulletin, Vol. 62 (1983), 904-915).

As a rule, in order to prepare polysilazanes, chlorosilanes are used asstarting materials and these are reacted with ammonia or primary orsecondary amines (U.S. Pat. No. 4,540,803, U.S. Pat. No. 4,543,344, U.S.Pat. No. 4,595,775, U.S. Pat. No. 4,397,828, U.S. Pat. No. 4,482,669).

The present invention provides novel starting materials forpolysilazanes, namely chlorine-containing silazane polymers.

The invention relates to a process for the preparation ofchlorine-containing silazane polymers, which comprises reactingoligosilazanes of the formula (I) ##STR2## in which a>0, b≧0 and n isabout 2 to about 12, with at least one of the chlorosilanes Cl₂ R⁴Si--CH₂ --CH₂ --SiR⁴ Cl₂, Cl₃ Si--CH₂ --CH₂ --SiR⁵ Cl₂, R⁶ SiCl₃ or R⁷SiHCl₂ at 30° C. to 300° C., where, independently of one another,

R¹ and R² =H, C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl,

R³, R⁴, R⁵, R⁶ and R⁷ =C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl and

where if b=0 the reaction with R⁷ SiHCl₂, if R⁷ =C₁ -C₆ -alkyl and noneof the other chlorosilanes is present, is to be excluded. It ispreferable to select a=0.85-0.98 and b=0.02-0.15.

The chlorine-containing silazane polymers prepared according to theinvention are alternatively also designated below as polymericchlorosilazanes.

The oligosilazanes used as starting materials may be obtained, byreacting a mixture of chlorosilanes of the formulae R¹ R² SiCl₂ and Cl₂R³ Si--CH₂ CH₂ --SiR³ Cl₂ in which R¹ to R³ have the above meaning, inn-pentane at -70° to +100° C. with an excess of NH₃, in a manneranalogous to that described in U.S. Pat. No. 4,482,669 formethyldichlorosilane (cf. particularly columns 4, 5, 7, 8 in the latterpatent). Generally, from this reaction, a mixture of linear and cyclicoligomers having different chain lengths n is formed.

Cl₂ R³ Si--CH₂ CH₂ --SiR³ Cl₂ is accessible by hydrosilylation from R³HSiCl₂ and ethyne.

Preferably, R¹ and R² =H, C₁ -C₃ -alkyl or C₂ -C₃ -alkenyl, and R³, R⁴,R⁵, R⁶ and R⁷ =C₁ -C₃ -alkyl or C₂ -C₃ -alkenyl, where the said radicalscan be identical or different. The case in which R¹ ═H, R² ═R³ ═R⁴ ═R⁵═R⁷ ═CH₃ and R⁶ ═CH₃ or vinyl is particularly preferred.

The reaction of oligosilazanes of the formula (I) in which b=0, (i.e.thus of oligosilazanes of the formula [R¹ R² --Si--NH]_(n) where n isabout 2 to about 12) with chlorosilanes of the formula R⁷ SiHCl₂ for thecase where R⁷ =C₁ -C₆ -alkyl and none of the other chlorosilanes Cl₂ R⁴Si--CH₂ CH₂ --SiR⁴ Cl₂, Cl₃ Si--CH₂ CH₂ --SiR⁵ Cl₂ and R⁶ SiCl₃ issimultaneously present has already been described in the German PatentApplication P 37 33 727.0 and is therefore not claimed in the presentapplication.

The molar ratio of the reactants chlorosilane:monomer unit of theoligosilazane (n=1) during the reaction to form the polymericchlorosilazane is preferably about 0.1:1 to about 1.5:1, particularlyabout 0.1:1 to about 0.7:1.

In order to bring the reactants into reaction with each other theoligosilazanes are preferably initially introduced and at least one ofthe chlorosilanes mentioned is added. Since the reaction is exothermic,the temperature is preferably initially kept at 30° to 50° C. whenadding the reactants together. The reaction mixture is subsequentlyheated to temperatures of 100° to 300° C., preferably to 120° to 250° C.

The NH₃ formed as a by-product partially escapes during the reaction.Once the reaction has reached completion, the remaining readily volatilecompounds are generally removed from the reaction vessel by applying avacuum.

The NH₄ Cl which is also formed during the reaction sublimes for themost part out of the reaction mixture in the course of the reaction. Anyresidual NH₄ Cl can be separated off from the polymeric chlorosilazaneprepared according to the invention by extraction with an inert organicsolvent, such as n-hexane, toluene or ether.

The duration of the reaction is governed by the rate of heating and thereaction temperature. Generally, a reaction time of 3 to 7 hours issufficient.

It is also possible to carry out the reaction in an organic solvent.Suitable solvents are those which are inert towards the reactants andhave a sufficiently high boiling point, that is to say, for example,saturated aliphatic or aromatic hydrocarbons such as n-decane, Decalin,xylene or toluene, chlorinated hydrocarbons such as chlorobenzene, orethers such as dibenzyl ether or diethylene glycol diethyl ether. If asolvent is used in which the NH₄ Cl formed is insoluble, the latter canbe separated off by filtration. The polymeric chlorosilazanes accordingto the invention are then obtained by distilling off the solvent underreduced pressure.

If appropriate, the process may also be carried out under reducedpressure. It is also possible to operate at pressures in the range of 1to 10 atmospheres.

The process can also be designed to operate continuously.

The novel polymeric chlorosilazanes prepared have the formula (II)##STR3## in which the free valencies of the nitrogen atoms are saturatedby H atoms or silyl radicals R*SiXN<(X=H,Cl,N<, CH₂ CH₂ Si←) and inwhich c, d, e, f, g and h denote the mole fractions of the individualstructural units and, independently of one another,

R¹ and R² =H, C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl

R⁴, R⁵, R⁶ and R*=C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl

R⁷ =C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl if at least one of the indices d,e, f and g>0, and

R⁷ =C₂ -C₆ -alkenyl if d=e=f=g=0.

The present invention also relates to these novel polymericchlorosilazanes. In these compounds, preferably R¹ and R² =H, C₁ -C₃-alkyl or C₂ -C₃ -alkenyl, and R³, R⁴, R⁵, R⁶ and R⁷ =C₁ -C₃ -alkyl orC₂ -C₃ -alkenyl. A particularly preferred case is that in which R¹ =H,R² =R³ =R⁴ =R⁵ =R⁷ =CH₃ and R⁶ =CH₃ or vinyl. Here, the above definitionof R⁷ is to be found.

The polymeric chlorosilazanes have a net-like structure. Whether themole fractions c to h assume positive values or the value 0 depends onthe oligosilazanes of the general formula (I) used in the reaction withthe chlorosilanes:

If (I) is reacted only with R⁶ SiCl₃, then e=f=h=0; c, d and g assumepositive values.

If (I) is reacted only with R⁷ HSiCl₂, then e=f=0; c, d, g and h assumepositive values, and R⁶ =R⁷.

If (I) is reacted only with Cl₂ R⁴ Si--CH₂ CH₂ --SiR⁴ Cl₂, then f=g=h=0;c, d and e assume positive values.

If (I) is reacted with a mixture of Cl₃ Si--CH₂ CH₂ --SiR⁵ Cl₂ and R⁶SiCl₃, then e=h=0; c, d, f and g are positive.

The particular values of c to h in each case can be determined byintegration of the ¹ H--NMR spectra and by elementary analysis.

Generally, c+d is 0.3 to 0.9 and c>0 and d≧0; generally, e, f, g and hare of the order of 0.01 to 0.4. Here, c+d+e+f+g+h=1.

Preferred polymeric chlorosilazanes are those in which c and d are 0.3to 0.8, particularly 0.3 to 0.5. The preferred values for e, f, g and hare 0.01 to 0.3, particularly 0.01 to 0.2. These values can be checkedby the analytical methods mentioned. The preferred values for c, d, e,f, g and h which have just been mentioned have proved particularlyadvantageous when a fiber is to be produced as the end product of thepyrolysis (after conversion of the polymeric chlorosilazanes intopolysilazanes).

The present invention also relates to polymeric chlorosilazanes whichare obtainable by reacting ologosilazanes of the formula (I) ##STR4## inwhich a >0, b≧0 and n is about 2 to about 12, with at least one of thechlorosilanes Cl₂ R⁴ Si--CH₂ --SiR⁴ Cl₂, Cl₃ Si--CH₂ --SiR⁵ Cl₂, R⁶SiCl₃ or R⁷ SiHCl₂ at 30° C. to 300° C., where, independently of oneanother,

R¹ and R² =H, C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl,

R³, R⁴, R⁵, R⁶ and R⁷ =C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl and where if b=0the reaction with R⁷ SiHCl₂, if R⁷ =C₁ -C₆ -alkyl and none of the otherchlorosilanes is present, is to be excluded.

In these compounds, preferably R¹ and R² =H, C₁ -C₃ -alkyl or C₂ -C₃-alkenyl, and R³, R⁴, R⁵, R⁵, R⁶ and R⁷ =C₁ -C₃ -alkyl or C₂ -C₃-alkenyl. The case in which R¹ =H, R² =R³ =R⁴ =R⁵ =R⁷ =CH₃ and R⁶ =CH₃or vinyl is particularly preferred.

The novel polymeric chlorosilazanes can be converted into polysilazanesby reaction with ammonia ("ammonolysis"), and the polysilazanes can inturn be converted by pyrolysis into ceramic material containing siliconnitride.

The ammonolysis can be carried out in liquid NH₃. However, it isadvantageous to carry out the ammonolysis in an organic solvent. Allsolvents which are inert towards the polymeric chlorosilazanes aresuitable. Preferred solvents are those in which the ammonium chlorideobtained as a by-product has low solubility and from which it can beeasily separated off, such solvents being, for example, ethers,aliphatic and aromatic hydrocarbons and chlorinated hydrocarbons. In theammonolysis, the reactants can be fed into the reaction vessel in anydesired order. However, it is usually advantageous initially tointroduce the polymeric chlorosilazane in solution and to pass ingaseous ammonia or to add liquid ammonia. If the polymericchlorosilazanes according to the invention have been prepared in asuitable organic solvent, the ammonolysis can be carried out in thissolvent without prior separation of the NH₄ Cl. The ammonolysis ispreferably carried out with an excess of NH₃, in order to ensure thatthe reaction is complete and the end products are as substantially freefrom chlorine as possible. Generally, double the stoichiometric amountis sufficient for this purpose.

Generally, the operation is carried out at a temperature of about -50°to +100° C., preferably at -20° to +30° C., particularly at roomtemperature (where ice is used for cooling). However, it is alsopossible to operate above room temperature, for example at the boilingpoint of the solvent used, or below room temperature, for example at-33° C. when liquid NH₃ is used.

After ammonolysis is complete, the excess of NH₃ is removed if necessaryand the ammonium chloride which has resulted is filtered off. In orderto increase the yield, the precipitate can be washed with one of theabove-mentioned organic solvents. The polysilazanes are obtaineddirectly as white powders after distilling off the solvent under reducedpressure. The polysilazanes are soluble in the above organic solvents,so that these can be used not only for coating surfaces but also forproducing fibers.

By pyrolysis in an inert atmosphere of nitrogen or argon at temperaturesof 800° to 1200° C., the polysilazanes can be pyrolysed to formamorphous, dense materials which are composed essentially of Si, N and Cand may also contain traces of H and O. At pyrolysis temperatures above1200° C., for example in the range from 1200° C. to 1400° C., partiallyamorphous, microcrystalline ceramic materials are produced, whichcontain α-Si₃ N₄ as a crystalline phase.

A particular advantage is that, before pyrolysis, the polysilazanes canbe shaped into three-dimensional shaped articles by various processes.

An important method of shaping is the drawing of fibers. By this method,fibers can be drawn from highly viscous solutions of the polysilazane insolvents such as toluene, THF or hexane. Fiber-drawing is advantageouslycarried out using spinning dies 80 to 150 μm in diameter. The thread isthinned out by subsequent stretching, so that after the pyrolysis, avery strong thread, 2 to 20 μm, in particular 5 to 15 μm, in diameter isproduced. The fibers which are produced by subsequent pyrolysis are usedas mechanical reinforcing inlays in fiber-reinforced aluminum, aluminumalloys and ceramic components.

A further important processing possibility with polysilazanes is theproduction of impervious, strongly adhering, amorphous ormicrocrystalline ceramic coatings on metals, in particular on steels.The coating is carried out with the aid of a solution of thepolysilazane in organic solvents such as toluene, THF or hexane. Thepyrolytic conversion into an amorphous or microcrystalline layer iscarried out in the same temperature range of from 800° to 1200° C. or1200° to 1400° C. under an inert gas, as described above in the case ofthree-dimensional shaped articles.

Due to their excellent adhesion, great hardness and surface quality, theceramic coatings are particularly suitable for the surface treatment ofmachine components which are subject to mechanical and chemical stress.

Moreover, it is possible to pyrolyse the polysilazanes described abovein an atmosphere of NH₃ instead of in an inert gas, with an equally highceramic yield of 70 to 90%. In this case, a glass-clear, colorlessmaterial results which is virtually carbon-free. When pyrolysis iscarried out in NH₃ at 1000° C. or above, the carbon content is below0.5% by weight. Depending on the pyrolysis temperature, the pyrolysisproduct is composed of virtually pure amorphous silicon nitride(pyrolysis below 1200° C.) or crystalline Si₃ N₄ (pyrolysis above 1200°C., in particular above 1300° C.). Pyrolysis in NH₃ can be employed forall those shaped articles produced by the shaping processes describedabove, namely articles, fibers and coatings shaped from powders.

Accordingly, the invention further relates to a process for thepreparation of a ceramic material containing silicon nitride, whichcomprises reacting the abovementioned polymeric chlorosilazanes, whichwe characterized by their formula or their process of preparation, withammonia at -50° to +100° C. and pyrolysing the polysilazane thus formedin an inert atmosphere of nitrogen or argon or in an atmosphere ofammonia at 800° to 1400° C.

Preferably, however, the conversion of the polymeric chlorosilazanesinto ceramic material containing silicon nitride is carried out in sucha way that the polysilazanes formed as intermediates are not isolated.In this case, the polymeric chlorosilazanes are preferably reacted withgaseous ammonia and the resulting reaction mixture is pyrolysed in anatmosphere of ammonia.

Accordingly, the present invention further relates to a process for thepreparation of ceramic material containing silicon nitride, whichcomprises reacting the abovementioned polymeric chlorosilazanes, whichare characterized by their formula or their process of preparation, withammonia at 0° to +300° C. and pyrolysing the reaction product in anatmosphere of NH₃ at 800°-1400° C.

EXAMPLES Example 1 Preparation of a Polymer of the Formula (II) byReaction of an Oligomer of the Formula (I) (a=0.67, b=0.33 and R¹ =H andR² =R³ =CH₃) with CH₃ SiCl₃

a. Preparation of the oligomer

60 ml (66.9 g, 0.58 mol) of CH₃ SiHCl₂ and 60 ml (72.0 g, 0.28 mol) ofCl₂ CH₃ Si--CH₂ CH₂ --SiCH₃ Cl₂ are dissolved in 1.5 l of absolute THFin a 2 liter four-necked flask fitted with a stirrer device, cooling andgas inlet device, and mixed. Gaseous ammonia was passed into thismixture at a rate such that the temperature of the reaction mixtureremained between 0° C. and 10° C. As soon as saturation had beenreached, i.e. all of the SiCl groups had been substituted by NH groups,the reaction mixture was allowed to thaw, in order subsequently toseparate off, under inert gas, the ammonium chloride which had beenformed during the reaction. The filtrate was freed from THF and theother volatile constituents under a reduced pressure of down to about 10mbar and at about 40° C. The solvent-free oligosilazane remained behindas a clear, highly mobile oil (62.7 g).

b. Preparation of the polymer

The oligosilazane was dissolved in 150 ml of toluene and 47 ml (59.8 g,0.4 mol) of CH₃ SiCl₃ were carefully added. The internal temperatureincreased to 52° C. during the addition. The mixture was then heatedunder reflux for 2 h, during which a colorless precipitate formed. Thesolvent was then distilled at reduced pressure into a cold trapmaintained at -78° C., the temperature of the oil bath being raised by10° C. every 15 min and the pressure inside the flask being reducedsomewhat, so that finally a pressure 0.1 mbar was reached at 220° C.During this operation some of the reaction mixture sublimed into thecooler parts of the vessel, and a clear melt remained behind. Oncooling, the latter became more viscous and finally solid, and at 20° C.the resulting substance had a glass-like brittleness and cleartransparency.

Yield: 45.6 g.

¹ H-NMR data: SiCH₃ +SiCH₂ CH₂ Si+NH: δ=0.0-2.0 ppm (broad), intensity20. SiH: δ=4.5-5.2 ppm (broad), intensity 0.9.

Analytical data: Found: Si 36.0%, N 16.0%, Cl 19.9%, 0<0.3%. Calculated:Si 37.2%, N 18.6%, Cl 17.1%, C 21.1%, H 4.8%.

The polymer had the formula (II) in which R¹ =H and R² =R³ =R⁶ =CH₃ :##STR5## with the values: c=0.38, d=0.19, g=0.43, e=f=h=0.

Example 2 Preparation of a Polymer of the Formula (II) by Reaction of anOligomer of the Formula (I) (a=0.75, b=0.25 and R¹ =H and R² =R³ =CH₃)with 1,2-bis(dichloromethylsilyl)ethane Cl₂ CH₃ Si--CH₂ CH₂ --SiCH₃ Cl₂

A procedure analogous to Example 1 was followed.

a. Preparation of the oligosilazane

Chlorosilanes used: 86.3 g (0.75 mol, 77.4 ml) of CH₃ SiHCl₂. 64.0 g(0.25 mol, 52.2 ml) of Cl₂ CH₃ Si--CH₂ CH₂ --SiCH₃ Cl₂,

b. Preparation of the polysilazane

The oligosilazane was reacted with 64 g (0.25 mol, 52 ml) of Cl₂ CH₃Si--CH₂ CH₂ --SiCH₃ Cl₂.

Yield: 69.3 g,

¹ H--NMR data: SiCH₃ +SiCH₂ CH₂ Si+NH: δ=0.0-2.1 ppm (broad), intensity14.4, SiH δ=4.5-5.1 ppm (broad), intensity 0.9.

Analytical data: Found: Si 35.3%, N 16.1%, Cl 17.1%, 0<0.3%. Calculated:Si 36.5%, N 15.2%, Cl 15.9%, C 25.5%, H 6.9%.

The polymer had the formula (II) in which R¹ =H and R² =R³ =R⁴ =CH₃ :##STR6## with the values: c=0.54, d=0.21, e=0.25 and f=g=h=0

Example 3 Preparation of a Polymer of the Formula (II) by Reaction of anOligomer of the Formula (I) (a=0.4, b=0.6 and R¹ =H and R² =R³ =CH₃)with a Mixture of Vinyltrichlorosilane and1,2-bis(dichloromethylsilyl)ethane

A procedure analogous to Example 1 was followed.

a. Preparation of the oligosilazane

Chlorosilanes used: 46 g (0.4 mol, 41.3 ml) of CH₃ SiHCl₂. 153.6 g (0.6mol, 125.2 ml) of Cl₂ CH₃ Si--CH₂ CH₂ --SiCH₃ Cl₂.

b. Preparation of the polysilazane

The oligosilazane was reacted with a mixture of 51.2 g (0.2 mol, 41.7ml) of Cl₂ CH₃ Si--CH₂ CH₂ --SiCH₃ Cl₂ and 16.3 g (0.1 mol, 12.8 ml) ofvinyl-SiCl₃.

Yield: 93.5 g.

¹ H--NMR data: SiCH₃ +SiCH₂ CH₂ Si+NH: δ=0.0-2.1 ppm (broad), intensity17.6, SiH: δ=4.5-4.9 ppm (broad), intensity 0.3, Si vinyl: δ=5.7-6.2 ppm(broad), intensity 0.5.

Analytical data: Found: Si 35.1%, N 18.4%, Cl 10.7%, 0<0.3%, Calculated:Si 36.8%, N 17.4%, Cl 9.4%, C 30.1%, H 7.3%.

The polymer had the formula (II) in which R¹ =H, R² =R³ =R⁴ =CH₃ and R⁶=vinyl: ##STR7## with the values: c=0.22, d=0.56, e=0.12 and f=h=0.

Example 4 Preparation of a Polymer of the Formula (II) by Reaction of anOligomer of the Formula (I) (a=0.5, b=0.5 and R¹ =R² =R³ =CH₃) with aMixture of ethyltrichlorosilane and1-trichlorosilyl-2-dichloromethylsilyl-ethane.

A procedure analogous to Example 1 was followed.

a. Preparation of the oligosilazane

Chlorosilanes used: 51.6 g (0.4 mol, 48.5 ml) of (CH₃)₂ SiCl₂. 102.4 g(0.4 mol, 83.5 ml) of Cl₂ CH₃ Si--CH₂ CH₂ --SiCH₃ Cl₂.

b. Preparation of the polysilazane

The oligosilazane was reacted with a mixture of 41.5 g (0.15 mol, 34.6ml) of Cl₃ Si--CH₂ CH₂ --SiCH₃ Cl₂ and 40.9 g (0.25 mol, 39.9 ml) of C₂H₅ SiCl₃.

Yield: 69.2 g.

¹ H--NMR data: SiCH₃ +SiCH₂ CH₂ Si+SiC₂ H₅ +NH δ=0.0-2.4 ppm (broad)

Analytical data: Found: Si 33.1%, N 17.4%, Cl 14.1%, 0<0.3%, Calculated:Si 34.6%, N 15.8%, Cl 13.6%, C 28.4%, H 6.9%.

The polymer had the formula (II) in which R¹ =R² =R³ =R⁵ =CH₃ and R⁶ =C₂H₅ : ##STR8## with the values: c=0.27, d=0.39, f=0.13, g=0.43 and e=h=0.

We claim:
 1. A process for the preparation of ceramic materialcontaining silicon nitride, which comprises reacting achlorine-containing silazane polymer of the formula (II) ##STR9## inwhich the free valencies of the nitrogen atoms are saturated by H atomsor silyl radicals R*SiXN<(X=H,Cl,N<, CH₂ CH₂ Si) and in which c, d, e,f, g and h denote the mole fractions of the individual structural unitsand, independently of one another,R¹ and R² =H, C₁ -C₆ -alkyl or C₂ -C₆-alkenyl R⁴, R⁵, R⁶ and R*=C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl R⁷ =C₁ -C₆-alkyl or C₂ -C₆ -alkenyl if at least one of the indices d, e, f andg>0, and R⁷ =C₂ -C₆ -alkenyl if d=e=f=g=0with ammonia at -50° to +300°C. and pyrolysing the reaction product thus formed in an inertatmosphere of nitrogen or argon or in an atmosphere of ammonia at 800°to 1400° C.
 2. A process as claimed in claim 1, which comprises reactinga chlorine-containing silazane polymer of said formula (II) with ammoniaat -50° to +100° C. and pyrolysing the polysilazane thus formed in aninert atmosphere of nitrogen or argon or in an atmosphere of ammonia at800° to 1400° C.
 3. A process as claimed in claim 1, which comprisesreacting a chlorine-containing silazane polymer of said formula (II)with ammonia at 0° to 300° C. and pyrolysing the reaction product in anatmosphere of NH₃ at 800°-1400° C.
 4. A process for the preparation ofceramic material containing silicon nitride which comprises reacting atleast one of the chlorine-containing silazane polymers (a) to (e)(a) achlorine-containing silazane polymer obtained by reacting anoligosilazane of the formula ##STR10## in which a>0, b≧0 and n is about2 to about 12, with at least one of the chlorosilanes Cl₂ R⁴ Si--CH₂--CH₂ --SiR⁴ Cl₂, Cl₃ Si--CH₂ --CH₂ --SiR⁵ Cl₂, R⁶ SiCl₃ or R⁷ SiHCl₂ at30° C. to 300° C., where, independently of one another,R¹ and R² =H, C₁-C₆ -alkyl or C₂ -C₆ -alkenyl, R³, R⁴, R⁵, R⁶ and R⁷ =C₁ -C₆ -alkyl orC₂ -C₆ -alkenyl and if b=0, and no other chlorosilanes are presented,then the reaction with R⁷ SiHCl₂, wherein R⁷ =C₁ -C₆ -alkyl is excluded;(b) a chlorine-containing silazane polymer obtained by reacting anoligosilazane of the formula ##STR11## in which a>0, b>0 and n is about2 to about 12, with at least one of the chlorosilanes Cl₂ R⁴ Si--CH₂--CH₂ --SiR⁴ Cl₂, Cl₃ Si--CH₂ --CH₂ --SiR⁵ Cl₂, R⁶ SiCl₃ or R⁷ SiHCl₂ at30° C. to 300° C., where, independently of one another,R¹ and R² =H, C₁-C₆ -alkyl or C₂ -C₆ -alkenyl, R³, R⁴, R⁵, R⁶ and R⁷ =C₁ -C₆ -alkyl orC₂ -C₆ -alkenyl; (c) a chlorine-containing silazane polymer obtained byreacting an oligosilazane of the formula ##STR12## in which n is about 2to about 12, with at least one of the chlorosilanes Cl₂ R⁴ Si--CH₂ --CH₂--SiR⁴ Cl₂, Cl₃ Si--CH₂ --CH₂ --SiR⁵ Cl₂, R⁶ SiCl₃ or R⁷ SiHCl₂ at 30°C. to 300° C., where, independently of one another,R¹ and R² =H, C₁ -C₆-alkyl or C₂ -C₆ -alkenyl, R⁴, R⁵, R⁶ and R⁷ =C₁ -C₆ -alkyl or C₂ -C₆-alkenyl wherein, if no other chlorosilanes are present, the reaction ofthe oligosilazanes with R⁷ SiHCl₂, where R⁷ =C₁ -C₆ -alkyl is excluded;(d) a chlorine-containing silazane polymer obtained by reacting anoligosilazane, which has been obtained by reaction of an excess ofammonia with a mixture of R¹ R² SiCl₂ and Cl₂ R³ Si--CH₂ CH₂ --SiR³ Cl₂at 30° C. to 300° C., with at least one of the chlorosilanes Cl₂ R⁴Si--CH₂ CH₂ --SiR⁴ Cl₂, Cl₃ Si--CH₂ CH₂ --SiR⁵ Cl₂, R⁶ SiCl₃ or R⁷SiHCl₂ at 30° C. to 300° C., where, independently of one another, R¹ andR² =H, C₁ -C₆ -alkyl or C₂ -C₆ -alkenyl and R³, R⁴, R⁵, R⁶ and R⁷ =C₁-C₆ -alkyl or C₂ -C₆ -alkenyl; or (e) a chlorine-containing silazanepolymer obtained by reacting an oligosilazane, which has been obtainedby reaction of an excess of ammonia with R¹ R² SiCl₂ at 30° C. to 300°C., with at least one of the chlorosilanes Cl₂ R⁴ Si--CH₂ CH₂ --SiR⁴Cl₂, Cl₃ Si--CH₂ CH₂ --SiR⁵ Cl₂, R⁶ SiCl₃ or R⁷ SiHCl₂ at 30° C. to 300°C., where, independently of one another,R¹ and R² =H, C₁ -C₆ -alkyl orC₂ -C₆ -alkenyl and R⁴, R⁵, R⁶ and R⁷ =C₁ -C₆ -alkyl or C₂ -C₆ -alkenylwherein, if no other chlorosilanes are present, then the reaction of theoligosilazanes with R⁷ SiHCl₂, when R⁷ =C₁ -C₆ -alkyl is excluded;withammonia at -50° to +300° C. and pyrolysing the reaction product thusformed in an inert atmosphere of nitrogen or argon or in an atmosphereof ammonia at 800° to 1400° C.
 5. A process as claimed in claim 4, whichcomprises reacting a said chlorine-containing silazane polymer withammonia at -50° to +100° C. and pyrolysing the polysilazane thus formedin an inert atmosphere of nitrogen or argon or in an atmosphere ofammonia at 800° to 1400° C.
 6. A process as claimed in claim 4, whichcomprises reacting a said chlorine-containing silazane polymer withammonia at 0° to 300° C. and pyrolysing the reaction product in anatmosphere of NH₃ at 800°-1400° C.